The present invention relates to the field of television signal transmission and, more particularly, is directed to a method and apparatus for creating a television signal and encrypting or decrypting the signal at the same time.
Television signals are produced and displayed as a result of a line scanning process. The picture information is scanned using a progressive series of horizontal lines which are transmitted sequentially in time. The transmitted signal is a continuous analogue of the brightness intensity corresponding to each point of the line. Such a signal is shown in FIG. 1 from which it may be seen that in a series of standard lines, any two adjacent active line periods (periods during which video information is transmitted) are separated by a period in which no video information is transmitted. This latter period is known as the line blanking interval and is introduced to allow the scanning device in the receiver to reset to the line-start position.
In typically color television signals, the active line period includes one signal which simultaneously represents the instantaneous values of three independent color components. The method by which the three color components are coded into one signal is standardized throughout North America, Canada and Japan. This method is known as the NTSC standard. Alternative standards known as PAL and SECAM have been adopted in other countries but these standards have the same basic format as the NTSC standard, including a line-blanking interval and an active line period in each scan line.
Other types of analogue video signals which are particularly adapted to transmission by satellite and cable, and which lead to improved picture quality in comparison with existing standards, are presently being studied. These signals are based on a time multiplex of the three independent color components during the active line period of the scan line. Instead of coding the three components into one signal using the NTSC, PAL or SECAM standard, the components are sent sequentially using a time-compression technique. One version of this type of signal is know as MAC (Multiplexed Analogue Components). Signals generated by a time comparison technique also adhere to the same basic format as the NTSC, PAL and SECAM standards, including the presence of a line-blanking interval and an active line period in each scan line. It should be noted that when a MAC signal is employed, digital data may also be transmitted during the line-blanking interval as shown by the dotted lines in FIGS. 2a and 2c.
Color video signals broadcast under the NTSC standard require that picture information be separated into two components: luminance, or brightness, and chrominance, or color. FIG. 10 is an amplitude-vs.-frequency diagram illustrating, in simplied form, a typical NTSC composite color television signal 50 comprising a luminance signal 52 and a chrominance signal 54. (A composite television signal is one in which chrominance information is carried on a subcarrier.) The signal occupies a nominal bandwidth of 6 MHz with the picture carrier 56 being 1.25 MHz above the lower end of the band. Luminance information is modulated directly onto picture carrier 56, while chrominance information is modulated onto color subcarrier 58 which is in turn used to modulate picture carrier 56. Color subcarrier 58 has a frequency of 3.579545 MHz, a standard established by the NTSC. (Audio information is carried on another subcarrier 40 lying near the upper edge of the band.)
The region labeled A in FIG. 10 is of particular importance for it represents overlap between the luminance 52 and chrominance 54 signals. Since separation of luminance and chrominance is accomplished by filtering a frequency-division multiplexed signal, overlaps such as A between the two signals lead to several problems. If, upon reception, complete separation between luminance and chrominance is desired, the necessary filtering will cause the loss of some of the information in both signals. On the other hand, if no loss of information can be tolerated, then one must accept interference between the luminance and chrominance signals. Moreover, since the various parts of the NTSC television signals are transmitted at different frequencies, phase shifts occurring during transmission will affect them differently, causing the signal to deteriorate. Also, the available color information is severely limited by the small color bandwidth permitted.
As discussed in commonly assigned pending application Ser. No. 652,926 filed Sept. 21, 1984, and herein incorporated by reference, the above-mentioned MAC standard was developed to overcome the problems associated with the NTSC standard. A MAC color television signal is illustrated in FIG. 11, which is an amplitude-vs.-time diagram of a single video line of 63.56 .mu.s duration. The first 10.9 .mu.s is in the horizontal blanking interval (HPI) 62, in which no picture information is transmitted. Following HBI 62 are chrominance signal 64 and luminance signal 66, either of which may be time-compressed. Between chrominance signal 64 and luminance signal 66 is a 0.28 .mu.s guard band 68, to assist in preventing interference between the two signals.
The MAC color television signal of FIG. 11 is obtained by generating conventional luminance and chrominance signals (as would be done to obtain a conventional NTSC or other composite color television signal) and then sampling and storing them separately. Luminance is sampled at a luminance sampling frequency and stored in a luminance store, while chrominance is sampled at a chrominance sampling frequency and stored in a chrominance store. The luminance or chrominance samples may then be compressed in time by writing them into the store at their individual sampling frequency and reading them from the store at a higher frequency. A multiplexer selects either the luminance store or the chrominance store, at the appropriate time during the active line period, for reading, thus creating the MAC signal of FIG. 11. If desired, audio samples may be transmitted during the HBI; these are multiplexed (and may be compressed) in the same manner as the video samples. The sample rate at which all samples occur in the multiplexed MAC signal is called the MAC sampling frequency.
Although the MAC format of FIG. 11 overcomes the problems of the composite television signal of FIGS. 1 and 10, there also exists in the prior art a need for secure encryption of video signals, such that only designated users may decrypt and display the information. In typical encryption systems, one or more parameters of the signal to be encrypted are modified according to a pattern which is determined at the transmitter. The pattern generally is a member of a large class of similar patterns, such that discovery of the pattern through exhaustive search is extremely unlikely. A precise description of the pattern used for encryption is delivered to designated receivers which then are able to recover the original information. The description of the pattern is known in the art as the "encryption key" and the process of informing designated users of the encrytion key is known as "key distribution."
With reference to FIG. 1, various encryption techniques known in the art will be described. As shown in FIG. 1, the video signal during the active line period may be represented by: EQU y=f(t)
where
y=amplitude (voltage) and PA1 t=time PA1 (1) Those which modify the amplitude (y) of the transmitted signal according to a prescribed pattern. EQU y'=g(f), PA1 f=f (t)
Knowledge of both the signal's amplitude (y) and the time at which it occurs (t) is necessary for accurate reconstruction of the video signal in a line scan system.
Encryption techniques may be classified as follows:
where
Examples of this technique include amplitude reversal of randomly chosen lines: EQU y'=g(f)=-f
(2) Those which modify the time at which the signal is transmitted through the channel: EQU y'=f(t')
Examples of this technique include the reordering of television lines according to a prescribed pattern: EQU y'=f(t-d)
(3) Those which modify both amplitude and transmission time.
It has been found that encryption techniques from the first category (variation of amplitude) cause distortion when the channel through which the signal is to be passed is non-linear. In this case, an amplitude (y) will be represented in the scrambled channel by various amplitudes according to the scrambling function in use at that instant. Channel non-linearity, therefore, causes imperfect reconstruction of the video information at the receiver. Since amplitude non-linearity is very common, it has been found that an optimum encryption algorithm should be selected from the second category, and, in particular, from the subset: EQU y'=f(t-d)
where d is constant during each standard line. In this case, the channel is subjected to an undistorted signal and only the time at which the signal occurs is scrambled. Since almost all channels are essentially `time invariant,` this technique introduces little distortion. This system is known as time-base scrambling.
An obvious method of time-base scrambling which has been used, is to reorder the television lines within the picture. This method, which results when d in the previous equation is an integral number of line periods, is complex, expensive and difficult to implement because recovery of the picture in the receiver demands storage of many television lines.